Postnatal development of synovial capillaries of rats with special reference to permeability

The size of a substance is a major factor determining whether it can permeate the wall of synovial capillaries. The maximum diameter of particles that can move across the synovial capillary wall has generally been thought to be 50 nm. We studied the permeability of the synovial capillaries of the rat between day 20 and 30 after birth using a polystyrene particle whose diameter was 240 nm. In addition using lecithin-coated polystyrene particles, we studied the maturation of the barrier function supported by endothelial and peripheral cells against foreign bodies. Lecithin-coated particles were found within the fibroblast-like synovial cells near the capillary in the 20 day-old rats, while non-coated particles remained in the endothelial wall and in the peripheral cells of capillaries. In the 30 day-old rats, lecithin-coated particles were present in the peripheral cells and the neighboring synovial cells; however, the non-coated particles were never found in the synovial or perisynovial cells. The present study shows that the size of the transportable substance by transcytosis may be larger than previously thought. Furthermore, the synovial capillaries functionally changed between day 20 and 30 suggesting that active movement of the joint led to the functional maturation of the synovial capillaries.

SDF-1 activity on microvascular endothelial cells: consequences on angiogenesis in in vitro and in vivo models

The chemokine stromal cell-derived factor-1 (SDF-1) has been shown to be involved in cell migration. As the receptor CXCR-4 is expressed on endothelial cells and upregulated by angiogenic factors, we were prompted to study the effect of SDF-1 on angiogenesis in endothelial cells from microvasculature. This study demonstrates that SDF-1 induces an angiogenic effect in vitro, primarily in a tridimensional fibrin gel. The increase in capillary tube formation was evident after a 10-day incubation with SDF-1. This was associated with a mild increase in VEGF production by microvascular endothelial cells (ELISA and rt-PCR) and a potent chemotactic effect. SDF-1 also induced an in vivo angiogenic activity as shown in the model of the rabbit corneal pocket. However, the angiogenesis was located in an area rich in inflammatory cells. The results of our study suggest that these data underline the potential role of SDF-1 in angiogenesis as the microvascular endothelial cells were greatly involved in this process.

Ultrastructural and morphometric studies related to expression of the cell adhesion molecule PECAM-1/CD31 in developing rat lung

It has recently been postulated that platelet/endothelial cell adhesion molecule-1 (PECAM-1/CD31) might play a role in vascular tube formation. To evaluate the role of PECAM-1/CD31 in the formation of the capillary network in vivo, we conducted an ultrastructural immunohistochemical evaluation of the localization of PECAM-1/CD31 and its developmentally regulated expression in the periphery of the lungs of fetal, newborn, and adult rats. PECAM-1/CD31 was present mainly on luminal surfaces and at the junctions between endothelial cells. Moreover, in fetal lung, products of the immunoreaction were also found on the abluminal surfaces of endothelial cells. To relate those findings to the developmental changes in the capillary area of the lung, we performed a morphometric study of electronmicrographs. The cross-sectional area of blood vessels at the periphery of the lungs was significantly greater in 15-19-day-old fetuses than in postpartum animals (p<0.0001). Disappearance of the expression of PECAM-1/CD31 on the abluminal endothelial surface paralleled the changes in the cross-sectional area of blood vessels that occurred during the perinatal period. (J Histochem Cytochem 48:1283-1289, 2000)

Capillary morphogenesis during human endothelial cell invasion of three-dimensional collagen matrices

Here, we describe assay systems that utilize serum-free defined media to evaluate capillary morphogenesis during human endothelial cell (EC) invasion of three-dimensional collagen matrices. ECs invade these matrices over a 1-3-d period to form capillary tubes. Blocking antibodies to the alpha2beta1 integrin interfere with invasion and morphogenesis while other integrin blocking antibodies do not. Interestingly, we observed increased invasion of ECs toward a population of underlying ECs undergoing morphogenesis. In addition, we have developed assays on microscope slides that display the invasion process horizontally, thereby enhancing our ability to image these events. Thus far, we have observed intracellular vacuoles that appear to regulate the formation of capillary lumens, and extensive cell processes that facilitate the interconnection of ECs during morphogenic events. These assays should enable further investigation of the morphologic steps and molecular events controlling human capillary tube formation in three-dimensional extracellular matrices.

[Therapeutic angiogenesis using genetic transfection. An in vitro quantitative and functional study after gene code transfer for vascular endothelial growth factor]

Vascular endothelial growth factor (VEGF) gene transfer is a new therapeutic approach in clinical situations where insufficient angiogenesis may impair processes that require vessel formation, such as myocardial ischemia or peripheral vascular disease. The feasibility of this strategy was addressed in vitro by (i) optimisation of DNA transfection into cultured cells using a cationic liposome, (ii) study of the transgene production and it's angiogenic properties. An expression plasmid encoding VEGF165 was used to transfect endothelial cells (EAhy-926) in vitro. Different amounts of the cationic liposome were complexed to the plasmid DNA in order to achieve increasing ratios (1/1, 2/1, 3/1, 4/1, 5/1). Maximal gene expression was obtained when the cationic liposome was mixed to plasmid DNA in a 3/1 ratio. Using this optimised liposome/plasmid DNA formulation, VEGF expression measured by ELISA, reached a maximum of 60 ng/mL 24 hours after gene transfer then rapidly decreased. Conditioned media from transfected cells (CMVEGF) significantly increased the proliferation rate of endothelial cells. The maximal mitogenic effect was observed when the cell media was supplemented with 25% of CMVEGF, corresponding to 10 ng/mL of recombinant VEGF. Addition of the VEGF165 peptide into the cell media showed a similar mitogenic effect. The angiogenic property of the transgene was assessed by the demonstration of its ability to stimulate capillary tubes formation in a tridimensional biologic gel.

Regulation of VEGF and VEGF receptor expression in the rodent mammary gland during pregnancy, lactation, and involution

Vascular endothelial growth factors (VEGFs) are endothelial cell-specific mitogens with potent angiogenic and vascular permeability-inducing properties. VEGF, VEGF-C, and VEGFRs -1, -2, and -3 were found to be expressed in post-pubertal (virgin) rodent mammary glands. VEGF was increased during pregnancy (5-fold) and lactation (15-19-fold). VEGF-C was moderately increased during pregnancy and lactation (2- and 3-fold respectively). VEGF levels were reduced by approximately 75% in cleared mouse mammary glands devoid of epithelial components, demonstrating that although the epithelial component is the major source of VEGF, approximately 25% is derived from stroma. This was confirmed by the findings (a) that VEGF transcripts were expressed predominantly in ductal and alveolar epithelial cells, and (b) that VEGF protein was localized to ductal epithelial cells as well as to the stromal compartment including vascular structures. VEGF was detected in human milk. Finally, transcripts for VEGFRs -2 and -3 were increased 2-3-fold during pregnancy, VEGFRs -1, -2 and -3 were increased 2-4-fold during lactation, and VEGFRs -2 and -3 were decreased by 20-50% during involution. These results point to a causal role for the VEGF ligand-receptor pairs in pregnancy-associated angiogenesis in the mammary gland, and suggest that they may also regulate vascular permeability during lactation.

[Therapeutic angiogenesis as a new experimental treatment for vascular disease]

Currently, our treatment modalities for patients with severe coronary artery disease consist of combinations of medication, percutaneous transluminal coronary angioplasty (PTCA) and coronary revascularization operations. Still, the number of patients who cannot be treated adequately in these ways is growing. In recent years progress has been made in the field of angiogenesis: the process of the development of new capillaries. It is now known that blood vessel growth is an essential phenomenon in a range of disease. It is possible to inhibit or stimulate this process, offering hope for new treatments in a wide array of diseases. Stimulation of angiogenesis has already been successful in animal models of chronic peripheral and myocardial ischaemia. Results of experimental treatments of coronary patients have been reported since 1998. 'Therapeutic angiogenesis' may evolve as our fourth treatment modality for the treatment of coronary artery insufficiency.

Aging of skeletal muscle: a 12-yr longitudinal study

The present study examines age-related changes in skeletal muscle size and function after 12 yr. Twelve healthy sedentary men were studied in 1985-86 (T1) and nine (initial mean age 65.4 +/- 4.2 yr) were reevaluated in 1997-98 (T2). Isokinetic muscle strength of the knee and elbow extensors and flexors showed losses (P < 0.05) ranging from 20 to 30% at slow and fast angular velocities. Computerized tomography (n = 7) showed reductions (P < 0.05) in the cross-sectional area (CSA) of the thigh (12.5%), all thigh muscles (14.7%), quadriceps femoris muscle (16.1%), and flexor muscles (14. 9%). Analysis of covariance showed that strength at T1 and changes in CSA were independent predictors of strength at T2. Muscle biopsies taken from vastus lateralis muscles (n = 6) showed a reduction in percentage of type I fibers (T1 = 60% vs. T2 = 42%) with no change in mean area in either fiber type. The capillary-to-fiber ratio was significantly lower at T2 (1.39 vs. 1. 08; P = 0.043). Our observations suggest that a quantitative loss in muscle CSA is a major contributor to the decrease in muscle strength seen with advancing age and, together with muscle strength at T1, accounts for 90% of the variability in strength at T2.

Homeobox B3 promotes capillary morphogenesis and angiogenesis

Endothelial cells (EC) express several members of the Homeobox (Hox) gene family, suggesting a role for these morphoregulatory mediators during angiogenesis. We have previously established that Hox D3 is required for expression of integrin alphavbeta3 and urokinase plasminogen activator (uPA), which contribute to EC adhesion, invasion, and migration during angiogenesis. We now report that the paralogous gene, Hox B3, influences angiogenic behavior in a manner that is distinct from Hox D3. Antisense against Hox B3 impaired capillary morphogenesis of dermal microvascular EC cultured on basement membrane extracellular matrices. Although levels of Hox D3-dependent genes were maintained in these cells, levels of the ephrin A1 ligand were markedly attenuated. Capillary morphogenesis could be restored, however, by addition of recombinant ephrin A1/Fc fusion proteins. To test the impact of Hox B3 on angiogenesis in vivo, we constitutively expressed Hox B3 in the chick chorioallantoic membrane using avian retroviruses that resulted in an increase in vascular density and angiogenesis. Thus, while Hox D3 promotes the invasive or migratory behavior of EC, Hox B3 is required for the subsequent capillary morphogenesis of these new vascular sprouts and, together, these results support the hypothesis that paralogous Hox genes perform complementary functions within a particular tissue type.

Density of perfused brain capillaries in the aged rat during the wake-sleep cycle

Structural and functional age-related changes in brain vasculature might affect the cerebral microcirculation. The present study evaluated the density of perfused brain capillaries and the perfusion fraction (perfused/existing capillaries) in aged rats (24 months) during the states of the wake-sleep cycle (quiet wakefulness, quiet sleep, active sleep) characterized by different levels of brain activation. The number of perfused capillaries was determined by intravascular injection of the fluorescent marker Evans Blue; histochemical staining of the capillary endothelium identified the alkaline-phosphatase (AP) reactive quota of the anatomical population. No sleep-related changes in perfused capillary density were found, and the perfusion fraction in the AP-stained sub-population was high and stable (95%) across the sleep-wake cycle: changing levels of brain activation during sleep do not affect functional capillary density in aged rats.

Inhibition of capillary growth in chronically stimulated rat muscles by N(G)-nitro-l-arginine, nitric oxide synthase inhibitor

Shear stress causes release of nitric oxide (NO) from microvascular endothelial cells in vivo and stimulates their growth in vitro. After chronic electrical stimulation of lower hind limb skeletal muscles in the rat, measurements of capillary diameters and red blood cell velocity indicated that shear stress is increased in these vessels as a potential source of NO. This study therefore investigated whether NO is involved in capillary growth in stimulated muscles. Control rats or those stimulated for 2 or 7 days were treated with the NO synthase inhibitor, N(G)-nitro-l-arginine (l-NNA, 10 mg.day(-1) in drinking water), or water alone. After bromodeoxyuridine (BrdU) administration, extensor digitorum longus muscles were removed and frozen. Capillary supply was assessed in cryostat sections as capillary:fiber (C:F) ratio after staining for alkaline phosphatase; proliferation of capillary-linked and interstitial nuclei was evaluated by immunostaining for BrdU incorporation. C:F was not increased after 2 days of stimulation but the increase after 7 days (1.88 +/- 0.50 vs control 1.45 +/- 0.04, P < 0.001) was abolished by l-NNA (1.55 +/- 0.04, NS). The labeling index for BrdU-positive nuclei colocalized with capillaries as a percentage of total interstitial nuclei increased in muscles stimulated for 2 days (11.3 +/- 2.2%) and 7 days (10.6 +/- 0.8%) compared with controls (2.9 +/- 0.5%, P < 0.01) and was eliminated by l-NNA at both time points (3.1 +/- 0.6 and 1.0 +/- 0.6%, respectively; both P < 0.05 vs stimulated). A transient increase in BrdU labeling of interstitial nuclei not associated with capillaries (possibly fibroblasts) after 2 but not 7 days stimulation was eliminated by l-NNA treatment. These results suggest that NO is involved in capillary growth in chronically stimulated muscles possibly via its shear-stress-induced release from capillaries or from interstitial fibroblasts.

B1 integrin activation inhibits in vitro tube formation: effects on cell migration, vacuole coalescence and lumen formation

Human endothelial cells (EC), when plated onto gels of extracellular matrix proteins such as Matrigel or collagen form capillary tubes in a process thought to mimic angiogenesis. We have shown previously that the extent of tube formation and the phenotype of the lumen are regulated by integrins (Gamble et al 1993) and lumen formation occurs through a process of vacuolization, coalescence and ultimate directional fusion of these vacuoles with the plasma membrane (Meyer 1997 et al). We now show here that activation of beta1 integrins on endothelial cells inhibits tube formation. On collagen gels, endothelial cells treated with 31 activating antibody 8A2 failed to migrate into the gel and tube formation was inhibited. Although several integrins mediate EC attachment to collagen alpha2beta1 is the chief determinant of EC behaviour since a blocking antibody to (alpha2beta1 reversed the effect of 8A2. On Matrigel tube formation was also inhibited by 8A2 treatment although cell alignment and sprout formation was still evident. Electron microscopy revealed the organisation of normal numbers of cells into solid sprouts and the formation of small intracellular vacuoles suggesting that initial stages of tube formation including cell migration were unaffected. However, beta1 integrin activation inhibited the coalescence of these small vacuoles into larger vacuoles, the recruitment of more cells into the sprout and the subsequent formation of mature lumen. The inhibition of capillary tube formation by beta1 activation was time dependent and long lasting. The critical time for activation of the beta1 integrin was the initial 1-2h after plating in order to inhibit tube formation although once activated, the beta1 mediated inhibition on Matrigel was still evident 4 days later. Our results suggest that beta1 integrins are critical in capillary tube formation in at least two phases. beta1 integrins are essential for migration of EC through collagen gels. Independently, beta1 integrins, although not involved in initial vacuole formation, are involved in the process of vacuole coalescence and subsequent lumen formation since beta1 integrin activation inhibits these processes.

Effect of thalidomide and structurally related compounds on corneal angiogenesis is comparable to their teratological potency

Neovascular diseases are the leading causes of blindness in humans. Although several promising compounds have been isolated, pharmacological treatment remains difficult. Thalidomide has inhibitory effects on angiogenesis in the corneal micropocket assay. However, the results vary considerably depending on the administration route and animal model. The aim of this study, therefore, was to investigate thalidomide and two of its derivatives, supidimide and EM12, in the rabbit corneal micropocket assay. Using both basic fibroblast growth factor and vascular endothelial growth factor for initiation of the neovascular response, we were able to show a significant inhibition of neovascularisation with all three substances. EM12, the most teratogenic derivative analysed, was demonstrated to be the most potent inhibitor of angiogenesis in this model. Thalidomide and supidimide did not show systemic side effects in the applied dosage. An equal dosage of EM12, however, resulted in significant weight loss of the animals, but did not increase angiogenic activity compared with lower doses. Together with earlier findings, these data support a strong correlation between the antiangiogenic potential and the teratogenic activity of thalidomide and structurally related compounds.

Developmental regulation of endothelial nitric oxide synthase in cerebral vessels of newborn pig by prostaglandin E(2)

We investigated whether prostaglandins regulate endothelial nitric oxide synthase (eNOS) in the pig cerebral vasculature during the neonatal period. Prostaglandins, eNOS mRNA, eNOS protein, and NO production were higher in cerebral microvessels of newborn (1 day old) than in those of adult (6- to 8-month-old) pigs. The treatment of isolated cerebral microvessels of newborn animals with ibuprofen for 24 h reduced eNOS mRNA and nitrite production to levels in the adult; this effect of ibuprofen was prevented by concurrent treatment with prostaglandin (PG)E(2) analog 16,16-dimethyl-PGE(2), nonselective PGE(2) receptor analog 11-deoxy PGE(1), and prostaglandin EP(3) receptor agonists sulprostone and M&B 28,767 but was not modified by PGI(2) analog carbaprostacyclin, PGD(2), and EP(1) receptor agonist 17-phenyl trinor PGE(2). Correspondingly, 16, 16-dimethyl-PGE(2) and M&B 28,767 increased eNOS mRNA expression of adult microvessels to values in the newborn. Data similar to those with isolated cerebral vessels were obtained through histochemical analysis (NADPH-diaphorase positivity) of brain from newborn animals treated in vivo with ibuprofen in combination or not with sulprostone. Furthermore, substance P-induced NO-mediated cerebral vasorelaxation was decreased to adult values through the treatment of newborn pigs with ibuprofen; this effect was prevented by concomitant treatment with sulprostone. It is concluded that PGE(2) regulates eNOS in newborn pig cerebral microvessels via EP(3) receptors; this may be physiologically required during normal neurovascular development.

Angiopoietin-1 and its receptor Tie-2 participate in the regulation of capillary-like tubule formation and survival of endothelial cells

Angiopoietin-1 (Ang-1) and its receptor Tie-2, a trans-membrane tyrosine kinase uniquely expressed by endothelial cells, are shown by null mutation studies to be essential to developmental angiogenesis. The phenotypic abnormalities in these knockout animals suggest that Tie-2 signaling is necessary for the maintenance and expansion of the primitive capillary network. We present in vitro evidence indicating that the Ang-1/Tie-2 system participates in the regulation of capillary tubule formation and is necessary for the survival of confluent endothelial cells. Although recombinant Ang-1, which induces Tie-2 phosphorylation, has no effect on the proliferation of endothelial cells, treatment of confluent adult bovine aortic endothelial cells (ABAE) cells grown on collagen gels with Ang-1 (100 ng/ml) causes the cells to migrate into the collagen gel and form capillary-like tubules. The tubule-forming effect of Ang-1 is similar to the effect caused by FGF-2. A soluble form of the Tie-2 extracellular domain, in fivefold molar excess, blocks Ang-1-induced tubule formation. Specific elimination of Tie-2 protein expression in cultured ABAE cells as a result of transfection with an antisense oligonucleotide causes cell death in a dose-dependent manner (IC(50) = 50 nM). The antisense treatment has no effect on cells that do not express Tie-2. Cells treated with antisense oligonucleotide show a sixfold increase in the rate of apoptosis as assessed by in situ end labeling of fragmented DNA. These findings are consistent with the view that Ang-1/Tie-2 signaling is essential for both angiogenesis and endothelial cell survival.

Morphometric analysis of postnatal lung development in a marsupial: transmission electron microscopy

Postnatal lung development in the tammar wallaby was investigated using transmission electron microscopy and stereological morphometry. Volume densities of interstitial, epithelial and endothelial tissue and capillaries in the parenchymal septa were measured as were surface densities of the airspaces and gas exchange capillaries. Absolute changes in these parameters were related to body mass. Three phases of development were identified. During the ectothermic period, in the first 70 days after birth when the lung was in the terminal air sac phase, the most marked change was an increase in volume density of septal interstitium. The transitional period between ectothermy and endothermy, between 70 and 180 days after birth, corresponded to the alveolar phase and was characterised by accelerated increase in air space surface area. Maturation of the parenchymal septa and establishment of the mature capillary system occurred largely after 180 days when endothermy was established. The anatomical diffusion factor in the tammar wallaby adult is similar to that for eutherians.

Neutralizing TGF-beta1 antibody infusion in neonatal rat delays in vivo glomerular capillary formation 1

BACKGROUND

The interruption of transforming growth factor-beta (TGF-beta) signaling with dominant negative type II TGF-beta receptors in bovine glomerular endothelial cells abrogates capillary morphogenesis in vitro, and genetic defects in the TGF-beta1 signaling cascade in mice and humans result in abnormalities of blood vessel morphology. This study sought to determine whether TGF-beta1 participates in renal glomerular capillary development in vivo.

METHODS

To inhibit TGF-beta1 action, neutralizing anti-TGF-beta1 IgG was infused intra-arterially into the suprarenal aorta of three-day-old rats, and the glomerular endothelial cell appearance was evaluated two days later by immunohistochemical detection of the endothelium-specific von Willebrand factor, in situ analysis of vascular endothelial growth factor receptor binding, and morphometric study of developing glomerular structures by transmission electron microscopy.

RESULTS

The infusion of neutralizing the TGF-beta1 antibody markedly reduced the invasion of comma- and S-shaped bodies by endothelial cells, and inhibited organization of endothelial cells into capillaries in these structures. In addition, capillary lumen formation and endothelial cell fenestration in developing cortical, but not in deep, already mature glomeruli were inhibited by neutralizing TGF-beta1 antibody. Seven days after TGF-beta1 antibody infusion, glomeruli appeared normal, and no reduction in glomerular number was observed.

CONCLUSIONS

These findings suggest that TGF-beta1 plays a critical role in the formation of glomerular capillaries during renal development in the rat, and that flattening and fenestration of glomerular capillaries require the action of TGF-beta1.

Functional roles for PECAM-1 (CD31) and VE-cadherin (CD144) in tube assembly and lumen formation in three-dimensional collagen gels

Various in vitro models have been described that emulate one or more of the processes involved in angiogenesis in vivo. In the present study endothelial cells were cultured in three-dimensional type I collagen lattices in the presence of a mixture of basic fibroblast growth factor, vascular endothelial cell growth factor, and phorbol myristate acetate. Under these conditions, the endothelial cells rapidly assemble into an interconnected network of tube-like structures with a high frequency of intercellular canals or lumens. The formation of the networks and lumens was completely blocked by cycloheximide and by actinomycin D. Monoclonal antibodies directed against CD31 or vascular endothelial cadherin (VE-cadherin) inhibited the formation of endothelial tubes. A subtle difference in the morphology of cells treated with anti-CD31 versus anti-VE-cadherin was noted; namely, cells incubated in the presence of CD31 antibodies were rounded or formed attenuated tube-like structures, both of which were characterized by a single, large intra- or intercellular vacuole. In contrast, tube formation by cells incubated in the presence of VE-cadherin antibodies was also impaired and, most notably, demonstrated a reduction in either vacuole formation or vacuole fusion, depending upon the monoclonal antibody used. We suggest that the two endothelial-junction-associated proteins, CD31 and VE-cadherin, play different roles in the process of tube formation. CD31 appears to be required for cell elongation, migration, and/or invasion in the gels as well as for cell-cell association to form the network structures. VE-cadherin also appears to be required for cell-cell association, but additionally appears to play some role in the process of vacuolization or vacuole fusion leading to intercellular lumen formation.

The demonstration of immunoreactive dystrophin and its developmental expression in perivascular astrocytes

Immunoreactivity of dystrophin family proteins was observed in the astrocytes of the adult and immature rat hippocampus and cerebral cortex by using Dys2, a monoclonal antibody recognizing both 427 kDa and short dystrophin isoforms. As revealed by light and electron microscopy, immunostaining of the ribosomal apparatus and of pericapillary endfeet was particularly pronounced in the adult. In the pericapillary astrocyte processes immunostaining appeared between postnatal days 10 and 20, and reached the intensity seen in the adult by postnatal day 30. In the pericapillary astrocyte process, the membrane facing the endothelial basal lamina was the earliest structure to show the immunoreaction. At later stages, the pericapillary astrocyte process was gradually filled up with immunoprecipitate. Findings suggest that dystrophins are expressed coinciding with the development of the blood-brain barrier, and it is assumed that they contribute to the formation of this system.

Developmental expression of ZO-1 antigen in the mouse blood-brain barrier

Tight junction biogenesis during blood-brain barrier development (BBB) in mesencephalon microvessels of mouse embryos of day 9, foetuses of day 15 and 19 and new-born (2-day-old) mice was examined by light and electron microscopy, using monoclonal antibodies recognizing the tight junction peripheral membrane protein ZO-1. A faint spot-like staining began to be recognizable under the light microscope in day 15 vessels in which the endothelial cells showed isolated fusion points between the external plasma membrane leaflets under the electron microscope. A stronger labelling was present in microvessels of day 19 foetuses and new-born animals when the endothelial tight junction appeared completely differentiated. In the immunogold study, gold particles were seen scattered throughout the cytoplasm of endothelial cells of day 15 foetuses. In day 19 foetuses and in the new-born mice, gold particles were located only at the cytoplasmic surfaces of the tight junctions. The results indicate that the ZO-1 protein is a specific molecular marker in the developing brain endothelial tight junctions and that its expression takes place parallel to BBB morphofunctional maturation.

Age-related changes in the microcirculation of skeletal muscle

The age-related reduction in exercise capacity is associated with a reduction in cardiac output and maximal oxygen consumption (VO2max). The loss of muscle mass explains a large portion of the age-related decline in VO2max. The capillary supply to a muscle fibre is primarily determined by its size, but also by its metabolic profile and the metabolic profile of surrounding fibres. Thus the age-related fibre atrophy and changes in the fibre type composition are expected to be accompanied by changes in the capillarisation. The exchange of oxygen, blood-borne energy sources, metabolites and heat between the blood and muscle tissue takes place in the microcirculation. Changes in the microcirculation may thus affect the functioning and viability of the muscle. The resting blood flow is minimally affected by age, but blood flow during or following exercise is generally reduced. This may in part be due to a reduced vasodilatory capacity and a decreased capillarisation. However, the coupling between capillary supply to a fibre and its metabolic profile or the profile of the surrounding fibres is maintained. There are some changes in ultrastructure of the endothelium. The age-related changes in the microcirculation are associated with a reduced VO2max and exercise capacity. The adaptability of the microcirculation is maintained at old age.

T lymphocytes induce endothelial cell matrix metalloproteinase expression by a CD40L-dependent mechanism: implications for tubule formation

Neovascularization frequently accompanies chronic immune responses characterized by T cell infiltration and activation. Angiogenesis requires endothelial cells (ECs) to penetrate extracellular matrix, a process that involves matrix metalloproteinases (MMPs). We report here that activated human T cells mediate contact-dependent expression of MMPs in ECs through CD40/CD40 ligand signaling. Ligation of CD40 on ECs induced de novo expression of gelatinase B (MMP-9), increased interstitial collagenase (MMP-1) and stromelysin (MMP-3), and activated gelatinase A (MMP-2). Recombinant human CD40L induced expression of MMPs by human vascular ECs to a greater extent than did maximally effective concentrations of interleukin-1beta or tumor necrosis factor-alpha. Moreover, activation of human vascular ECs through CD40 induced tube formation in a three-dimensional fibrin matrix gel assay, an effect antagonized by a MMP inhibitor. These results demonstrated that activation of ECs by interaction with T cells induced synthesis and release of MMPs and promoted an angiogenic function of ECs via CD40L-CD40 signaling. As vascular cells at the sites of chronic inflammation, such as atherosclerotic plaques, express CD40 and its ligand, our findings suggest that ligation of CD40 on ECs can mediate aspects of vascular remodeling and neovessel formation during atherogenesis and other chronic immune reactions.

Distinct signal transduction pathways are utilized during the tube formation and survival phases of in vitro angiogenesis

Angiogenesis, the formation of new blood vessels from pre-existing ones, occurs during development, wound healing and cancer and involves stages that orchestrate a network of cooperative interactions. Peptide growth factors and extracellular matrix (ECM) components are two major groups of angiogenesis mediators. Among the different ECM proteins, collagens have been well-associated with in vivo angiogenesis. Using human umbilical vein endothelial cells (HUVEC) grown in 3-D collagen gels we show that: (1) HUVEC do not survive well in 3-D collagen gels due to rapid induction of apoptosis. (2) VEGF, a potent in vivo angiogenic factor, fails to induce tube formation. (3) PMA was effective in inducing tube formation and survival in HUVEC dispersed in 3-D collagen gels, activating MAP kinase, phosphoinositide 3-OH kinase (PI-3-kinase) and Akt/PKB (protein kinase B) pathways. (4) VEGF was effective in preventing PMA-induced tube-like structure regression after PMA-withdrawal by (5) activating the mitogen activated protein kinase (MAPK), rather than the Akt/PKB, signaling pathway.

Capillary growth in relation to blood flow and performance in overloaded rat skeletal muscle

Rat extensor digitorum longus muscles were overloaded by stretch after removal of the synergist tibialis anterior muscle to determine the relationship between capillary growth, muscle blood flow, and presence of growth factors. After 2 wk, sarcomere length increased from 2.4 to 2.9 micrometers. Capillary-to-fiber ratio, estimated from alkaline phosphatase-stained frozen sections, was increased by 33% (P < 0.0001) and 60% (P < 0.01), compared with control muscles (1.44 +/- 0.06) after 2 and 8 wk, respectively. At 2 wk, the increased capillary-to-fiber ratio was not associated with any changes in mRNA for basic fibroblast growth factor (FGF-2) or its protein distribution. FGF-2 immunoreactivity was present in nerves and large blood vessels but was negative in capillaries, whereas the activity of low-molecular endothelial-cell-stimulating angiogenic factor (ESAF) was 50% higher in stretched muscles. Muscle blood flows measured by radiolabeled microspheres during contractions were not significantly different after 2 or 8 wk (132 +/- 37 and 177 +/- 22 ml. min-1. 100 g-1, respectively) from weight-matched controls (156 +/- 12 and 150 +/- 10 ml. min-1. 100 g-1, respectively). Resistance to fatigue during 5-min isometric contractions (final/peak tension x 100) was similar in 2-wk overloaded and contralateral muscles (85 vs. 80%) and enhanced after 8 wk to 92%, compared with 77% in contralateral muscles and 67% in controls. We conclude that increased blood flow cannot be responsible for initiating expansion of the capillary bed, nor does it explain the reduced fatigue within overloaded muscles. However, stretch can present a mechanical stimulus to capillary growth, acting either directly on the capillary abluminal surface or by upregulating ESAF, but not FGF-2, in the extracellular matrix.